Superplastic deformation mechanisms of high Nb containing TiAl alloy with (α2 + γ) microstructure. (August 2016)

• Record Type: Journal Article
• Authors: Cheng, Liang; Li, Jinshan; Xue, Xiangyi; Tang, Bin; Kou, Hongchao; Bouzy, Emmanuel
• Is Part Of: Intermetallics. Volume 75(2016:Aug.)
• Page Start: 62
• Page End: 71
• Publication Date: 2016-08
• Subjects: Titanium aluminides, based on TiAl -- Superplastic behaviour -- Micro-texture -- Microstructure; Intermetallic compounds -- Metallography -- Periodicals; Metallic glasses -- Periodicals; Composés intermétalliques -- Métallographie -- Périodiques; 669.94
• DOI: 10.1016/j.intermet.2016.06.003
• Languages: English
• ISSNs: 0966-9795

Abstract

Abstract: In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (α2 + γ) microstructure, Ti–43.5Al–8Nb–0.2W–0.2B (at.%), has been examined and studied by means of hot tension from 850 °C to 1050 °C under an initial strain rate of 10 −4 s −1 . The mechanical behaviour and microstructure evolution have been characterized and analyzed. Besides, to gain insight into deformation mechanisms, the texture evolution during deformation at ordinary (non-superplastic) and superplastic conditions has been systematically studied. The results showed that, the alloy exhibited impressive superplastic elongation at 1000 °C with a strain-rate sensitivity exponent ( m ) of about 0.5 and an apparent activation energy ( Q app ) value of about 390 kJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 °C), severe grain refinement occurred and the fraction of low-angle grain boundary notably increased. Meanwhile, the textures were characterized by <100> and <111> double-fiber components parallel to the tensile direction. All these observations suggested a dislocation slip and twinning mechanism. However, if deformed at the superplastic condition (1000 °C), it was found that the microstructure was fairly stable in terms of grain size, morphology and grain boundary characteristics during tension, but a continuous weakening of the initial <110> fiber texture (resulted from canned-forging) was observed. This was believed to be an indication of grain boundary sliding mechanism. Moreover, the deformation texture (<100> + <111>)—though is very weak—was simultaneously appeared. According to a detailed discussion on the deformation kinetics and microstructure evolution, it was believed that the slip/twinning-accommodated grain boundary sliding was responsible for superplastic deformation and the dislocation climb inside of γ grains was the rate-controlling step. Graphical abstract: Highlights: The superplastic deformation mechanism was revealed by texture evolution. The rate-controlling mechanism was found to be lattice dislocation climb. A unified rate-equation for various microcrystalline TiAl alloys was developed.